This invention relates to linear array antennas aligned transverse to a forward beam direction and, more particularly, to such antennas utilizing a linear array of multi-radiator elements each of which includes two or more radiators in an end-fire configuration.
Identification Friend or Foe (IFF) systems are used to enable aircraft to transmit and receive signals for identification of other aircraft. Airborne radar systems are also used for target location without identification capabilities. The higher frequencies typically used for airborne radar permit use of antennas providing reasonable beam resolution both vertically and horizontally. Airborne linear array antennas used for IFF may, by contrast, lack the capability of providing vertical resolution. Without vertical, or elevation, resolving capability, no elevation information is provided by the system. Consider the example of a linear array antenna arranged to provide a vertical fan beam scannable side-to-side in azimuth. The straight vertical fan beam that the antenna provides in the on-boresight direction perpendicular to the linear array becomes curved or conical in shape when the beam is scanned off boresight. As a result, as illustrated in FIG. 1, if a target exists at a location (a) (15.degree. right and at the same altitude as the reference aircraft) the IFF display would accurately indicate a target at 15.degree. right. If however, a target were at location (b) (again 15.degree. right, but at a higher altitude) the IFF display would indicate a target at azimuth (c), displaced from the actual 15.degree. position of the target. The error is introduced by a "coning" of the antenna beam as it is scanned to the right and effectively assumes a profile of a form shown by curved beam profile (d). The resulting errors introduced by off-boresight coning of the IFF beam, in addition to affecting the accuracy of the IFF target display, can introduce a displacement between the IFF and radar returns displayed for the same target.
Although a linear array antenna arranged to use monopulse techniques does not produce a scannable fan beam, it can provide target azimuth information. However, when the target has an elevation bearing other than zero degrees, the azimuth information will be subject to the same azimuth errors as discussed with reference to FIG. 1.
The present inventor's prior U.S. Pat. No. 5,214,436, titled "Aircraft Antenna with Coning and Banking Correction", covers antennas providing coning correction by steerable beam configurations using multi-radiator elements having an effective center of radiation which can be shifted forward and backward. The full disclosure of U.S. Pat. No. 5,214,436 is hereby incorporated herein by reference.
Objects of the present invention are to provide new and improved linear array antenna systems employing coning error correction and such systems having one or more of the following advantages and characteristics:
provision of an azimuth correction factor representative of coning error in signal reception from an off-boresight target; PA1 correction of target azimuth by use of an azimuth correction factor; PA1 absence of requirement for modification of a linear array configuration; PA1 absence of requirement for separate additional antenna; PA1 use of an available antenna output signal which is normally resistively dissipated; and PA1 use of signals typically available in monopulse signal processing configurations. PA1 (a) providing a linear array of multi-radiator elements transverse to a boresight axis, each of such elements including at least a front radiator and a rear radiator; PA1 (b) providing, via an excitation circuit coupled to the elements, output signals representative of signals received from a distant signal source positioned off the boresight axis, including (i) a first output signal having an amplitude which is higher during reception from an on-boresight source than from an off-boresight source, and (ii) a second output signal having an amplitude which is lower during reception from an on-boresight source than from an off-boresight source over a range of angles; PA1 (c) comparing the amplitude of the first and second output signals to develop an azimuth correction factor for the distant signal source; PA1 (d) utilizing output signals provided in step (b) to determine an apparent azimuth bearing of the distant signal source by monopulse techniques; and PA1 (e) applying the azimuth correction factor to correct the apparent azimuth bearing determined for the distant signal source.